The object of wind power consists of generating electricity from the wind by means of wind turbines with the maximum efficiency and the minimum cost possible. Said wind turbines consist basically of: a tower; a nacelle accommodating an electrical generator; and a rotor formed by at least two blades. The tower is in charge of supporting all the elements included in the nacelle and transmitting to the foundation all the reaction forces generated as a consequence of the different aerodynamic actions and operation conditions of the wind turbine.
Generally, given the high dimensions of the wind turbine and, consequently, of the tower, the tower is made by assembly of precast reinforced concrete modular sections called dowels.
In some structural solutions, the dowels are subject to a post-tensioning by means of a series of inner steel cables in order to improve their mechanical capacities. This post-tensioning causes each section of the dowels work (in most of the operating cases of the turbine) at a higher compression level, limiting the traction cases, taking the advantage of the fact that the concrete compressive strength is significantly higher than its tensile strength. Furthermore, the compression process tends to close any pre-existing cracking or fault in the concrete, thus limiting the progression of said cracking or fault through the concrete. According to what has been stated above, there is an interest in post-tensioning the dowels and the joints between the tower sections.
A first option, commonly used, to perform post-tensioning, is shown in FIGS. 3 and 4 of the US patent application US20120141295. Said figures show a so called “section post-tensioning”, according to which, the tendons are housed in the dowels, so they have to pass through holes drilled in the dowels.
In this first option the assembly process is endangered since the cables have to be passed through a dowel towards the adjacent dowel, so that, given the high weight and dimensions of the dowels, manipulation thereof with respect to their positioning for that “threading” process, increases the assembly times of the wind turbine assembly.
A second option, also commonly used nowadays, to perform the above mentioned post-tensioning, comprises the use of steel cables, also referred to as tendons, which run longitudinally inside the tower, being those cables fixed to the tower foundation at an end, whereas at the other end they are fixed either to the upper part of a dowel or to a nacelle element referred to as upper flange.
The upper flange is usually made of steel and is fastened both to a yaw bearing (also referred to as nacelle upper bearing), and to nacelle driving sub-systems. Therefore, the task of post-tensioning the cables under these circumstances is an operation for which the space limitations imposed by said yaw bearing are critical.
On the other hand, the second option described implies the need to perform an effective pre-loading over the bigger dowels, located nearer the tower base, which implies the need of a bigger quantity of steel in the tendons, with respect to the post-tensioning section by section explained in the first option.
Therefore, performing a post-tensioning by sections implies a greater flexibility in terms of the cable sections and the number thereof, being possible to make an optimum use of said steel cables for each tower section.
However, said post-tensioning by sections poses the disadvantage of requiring a bigger number of operations of pre-loading of section cables, as well as the above reported inherent difficulty in manipulation of dowels (threading or insertion of the cables in the adjacent dowel housings).